Hi Everyone,
Sorry, but this issue was definitely due to ageing on the SIM card slots as the hardware we were using was more than a few years old and the connectivity on the slots was failing more than usual and was no longer reliably working all the time after multiple reboots. We ended up replacing the device which was an embedded system with a dedicated modem which are better designed to handle modem chips and SIM cards.
We attempted to repair the slots or remove any rust or dust that may have built up by putting some IsoPropyl Alcohol on a cotton bud and swabbing that inside the slots. This worked only for a while then we were getting SIM missing again.
Some things for anyone else experiencing this issue to try:
- Cleaning the SIM card slots/replacing them if possible
- Deconstructing the hardware and fault testing with a multimeter to follow the voltage signals from the modem to the SIM card slot
- Upgrading the SIM firmware/Upgrading Modem manager/Upgrading base OS
- Trying different SIM cards
- Verify all SIM settings on the modem by using modem AT commands
- Create a watchdog script which will reboot the modem or the device if it detects SIM missing to try and recover it
- For better control of the modem for recovery, make sure the main program used to drive the SIM is using the base modem AT commands rather than a wrapper like modem manager. The more control you have over the modem the better it will handle errors and try to recover from faults.